Methodology to Assess Minimum Accident of Concern and Criticality Accident Alarm System Location

Transcription

1 Methodology to Assess Minimum Accident of Concern and Criticality Accident Alarm System Location Ruxandra Dranga and Jingjing Wang ANS NCSD 2013 Criticality Safety in the Modern Era: Raising the Bar Wilmington, North Carolina 2013 Sept 29 Oct 3 UNRESTRICTED / ILLIMITÉ

2 Outline Motivation Minimum Accident of Concern (MAC) Definition General Description Methodology - Implementation Example Concluding Remarks UNRESTRICTED / ILLIMITÉ 2

3 Motivation Canadian Nuclear Safety Commission (CNSC) Regulatory and Guidance Documents on Nuclear Criticality Safety (RD-327 and GD-327) requirements: Define the minimum criticality accident of concern Identify possible criticality accident alarm system detector locations Evaluate which detector locations are appropriate to detect the minimum accident of concern CRL s new approach complies with newly updated regulations UNRESTRICTED / ILLIMITÉ 3

4 Minimum Accident of Concern (MAC) Definition As per CNSC RD-327, the MAC is defined as: the accident resulting in a dose to free air of 0.20Gy (20 rad) in the first minute, at a distance of 2 meter from the reacting material. (consistent with ANSI/ANS 8.3) Notes: if significant shielding was present around the accident, the MAC will account for the additional shielding UNRESTRICTED / ILLIMITÉ 4

5 Methodology Step 1: Define the Minimum Accident of Concern Step 2: Assess detector dose rate at each location UNRESTRICTED / ILLIMITÉ 5

6 General Description Computer code used: MCNP5, version 1.40 Neutron and photon coupled mode (i.e., MODE N P) was used for all calculations Criticality accident modeled as a criticality source using the KCODE card Critical systems were defined such that the calculated k eff was between and KCODE calculation was run until the tally results had < 1% relative uncertainty The energy deposited in air was calculated using the F6 tally, which calculates the energy deposited, in MeV / g / source particle, averaged over a tallied volume UNRESTRICTED / ILLIMITÉ 6

7 Step 1: Define the MAC 1 a) Identify the type(s) of fissile materials and postulate the conceivable sequence of events which would result in an inadvertent critical assembly 1 b) Perform a parametric study to estimate: The minimum critical mass MCM for MAC The optimal conditions (i.e., optimal light-water moderation, most conservative geometry given available, engineering controls, etc.) UNRESTRICTED / ILLIMITÉ 7

8 Step 1: Define the MAC 1 c) Model the postulated accident in MCNP to calculate the energy deposited in a 5 cm shell of air, located at 2 m away from the critical assembly The energy deposited includes both energy deposited by neutrons and gammas UNRESTRICTED / ILLIMITÉ 8

9 Step 1: Define the MAC 1 d) Calculate the total fission rate for each postulated accident: Energy deposited = the MCNP results obtained using the F6 tally, reported in MeV / g / source neutron eposited # fissions = the average ( Eq. 2) number of neutrons produced per accident fission, calculated using MCNP UNRESTRICTED / ILLIMITÉ 9

10 Step 1: Define the MAC 1 e) Identify the MAC as the accident resulting in the lowest fission yield Postulated Accident Description Estimated Fission Yield (# fissions / min) Bare sphere, homogeneous HEU - water mixture Reflected sphere, homogeneous HEU water mixture UNRESTRICTED / ILLIMITÉ 10

11 Step 2: Establish the location, type and # of detectors 2 a) Create a computer model of the nuclear facility UNRESTRICTED / ILLIMITÉ 11

12 Step 2: Establish the location, type and # of detectors 2 b) Identify the potential locations for the MAC defined in Step 1 UNRESTRICTED / ILLIMITÉ 12

13 Step 2: Establish the location, type and # of detectors 2 c) Select the location of potential detectors. Potential Detector Locations UNRESTRICTED / ILLIMITÉ 13

14 Step 2: Establish the location, type and # of detectors 2 d) Calculate neutron and gamma doses for each detector location: eposited dose rate = calculated neutron or gamma dose rate at the detector location Energy deposited = the MCNP results obtained using the F6 tally, in MeV/g/source neutron MAC fission rate = the term calculated in Step 1 d) # fissions = the ( Eqaverage. 2) number of neutrons produced per fission accident UNRESTRICTED / ILLIMITÉ 14

15 Step 2: Establish the location, type and # of detectors 2 e) Compare dose rate to detector trip point and establish which identified locations provide sufficient coverage for the MAC Analysis showed that detector locations 1 and 2 were sufficient to provide coverage for the four MAC locations Calculated gamma doses were bounding; hence, using gamma CAAS detectors was appropriate Implementing at least a 2-out-of-3 logic at each detector location is recommended to reduce the rate of false alarms UNRESTRICTED / ILLIMITÉ 15

16 Concluding Remarks First step in complying with CNSC regulatory requirements for criticality accident alarm system and in establishing the system parameters and design requirements Implemented a step-by-step process to establish the minimum accident of concern and identify criticality accident alarm detector location, type and number The results provide the basis for the acquisition, installation and/or upgrade of criticality accident alarm systems in facilities where a criticality accident may lead to an excessive radiation dose UNRESTRICTED / ILLIMITÉ 16

17 Acknowledgements The authors gratefully acknowledge the help and assistance provided by fellow staff at AECL, including Fred Adams, John Allen, Laura Blomeley, Lloyd Dunn, Darren Radford, Bruce Wilkin, and Ross Webster. UNRESTRICTED / ILLIMITÉ 17

18 UNRESTRICTED / ILLIMITÉ 18